This invention relates to an inverter power generation apparatus comprising an AC generator driven by an internal combustion engine at various rotational speeds and an inverter to convert an output of the AC generator into an AC output of arbitrary frequency and a method of controlling the inverter power generation apparatus on its overload.
An inverter power generation apparatus has been used for a power generation apparatus driven by an internal combustion engine as a primer.
In general, the inverter power generation apparatus comprises an AC generator driven by the internal combustion engine, a DC power supply section to convert an output voltage of the AC generator into a DC voltage and an inverter to convert an output voltage of the DC power supply section into an AC output of predetermined frequency.
The inverter comprises a bridge type switch circuit to convert the output of the DC power supply section into the AC voltage, a filter circuit to remove a harmonic component from the AC voltage output from the switch circuit, load connection terminals having the output of the filter circuit applied across the terminals and PWM control means to control the switch elements of the switch circuit in the form of pulse width modulation (referred to as PWM later) so as to output the AC voltage of predetermined waveform through the load connection terminals.
The DC power supply section comprises a rectifier to rectify the output of the AC generator and a smoothing capacitor connected across the DC output terminals of the rectifier to generate a DC voltage across the smoothing capacitor.
The bridge-type switch circuit comprises a plural of switch arms connected in parallel to each other and each having an upper arm switch element and a lower arm switch element connected in series to each other and an upper arm feedback diode and a lower arm feedback diode connected in reverse parallel to the upper arm switch element and the lower arm switch element, respectively. A pair of DC input terminals are led out of a common connection point of both ends of the switch arms while AC output terminals are led out of the connection point of the upper and lower arm switch elements of each of the switch arms.
The PWM control means to control the switch elements of the switch circuit serves to output from the inverter circuit the AC voltage of intermittent waveform having a duty ratio xe2x80x9cDxe2x80x9d changed for every PWM cycle in accordance with an instant value of the AC output voltage applied through the load connection terminals to the load by carrying out the on-off of the current flowing through the pair of switch elements located at the diagonal position of the bridge type switch circuit, for example at predetermined timing with a drive signal (PWM signal) of pulse waveform obtained by modulating in the PWM form at least one of drive signals to be applied to the pair of the switch elements.
The AC voltage of intermittent waveform output from the switch circuit is converted into an AC output voltage of smooth waveform having a harmonic component removed by the filter circuit.
In the inverter generator apparatus, there is provided overload protection means to stop the operation of the inverter when the load current gets excessive in order to protect the switch circuit of the inverter from the overload current. The overload protection means of the prior art inverter generator apparatus comprises a current transformer to detect the load current flowing from the inverter through the load connection terminals, an overload signal generation circuit to generate an overload signal when the load current detected by the current transformer is compared with a limit value and the detected overload current exceeds the limit value and inverter operation stop means to stop the operation of the inverter by stopping supplying the drive signal to the switch circuit of the inverter when the state where the overload signal is generated continues for a setting time.
With the inverter generator apparatus used, since the DC voltage output from the DC power supply section is converted into the AC voltage of arbitrary frequency, the AC voltage of predetermined frequency can be obtained from the load connection terminals in spite of the revolutional speed of the generator. Also, the AC voltage of arbitrary magnitude can be obtained by controlling the duty ratio changed for every PWM cycle in the PWM control.
As aforementioned, in the prior art inverter generator apparatus, the overload protection control is performed by stopping the operation of the inverter when the state where the load current flowing from the inverter through the load connection terminals exceeds the limit value continues for the setting time whereby the switch elements of the switch circuit of the inverter are protected from the over current, but since the operation of the inverter is stopped by the overload protection control working with the large rush current flowing through an induction load such as an induction motor when it starts to be driven, the load might be unable to be started.
In order to solve the problems, it will be considered that the limit value of the load current in the overload protection control is set to be higher than the rush current of the induction motor, but since the thus set limit value prevents the protection operation from being made even though the over current flows for a long time when the load other than the induction load is driven whereby the inverter cannot be positively protected.
Accordingly, it is a principal object of the invention to provide a method of controlling an inverter power generation apparatus on its overload adapted to positively protect an inverter used for both of an induction load and a load other than the induction load.
It is another object of the invention to provide an apparatus for controlling an inverter power generation apparatus on its overload adapted to positively protect an inverter used for both of an induction load and a load other than the induction load.
The invention is applied to a method of overload controlling an inverter power generation apparatus comprising an AC generator driven by an internal combustion engine, a rectifier to rectify an output of the AC generator and an inverter to convert an output voltage of the rectifier into an AC voltage of constant frequency. In the overload control method of the invention, a value of the load current flowing through the inverter is compared with an allowable threshold value and at least one overload judgment value set to be smaller than the allowable threshold value and the operation of the inverter is stopped immediately when the overload current exceeds the allowable threshold value. Also, in case that the load current is equal to or less than the allowable threshold value, but exceeds the overload judgment value, when a time set in accordance with the value of the overload judgment values elapses after the overload current exceeds the overload judgment value, the operation of the inverter is stopped.
Thus, the induction load through which the large current flows when it starts to be driven can be driven without any trouble by appropriately setting the overload judgment value and the times determined in accordance with the overload judgment values.
In the state of driving the induction load and the load other than the induction load, when the load current gets excessive and exceeds the allowable threshold value, the operation of the inverter can be immediately stopped. Thus, the inverter can be positively protected from the overload.
An inverter power generation apparatus for carrying out the aforementioned method may comprise a load current detector to detect a load current flowing through an inverter, an instant stop command generation circuit to generate a stop command to instruct the inverter to be stopped immediately when the value of the load current detected by the load current detector is compared with an allowable threshold value and the load current exceeds the allowable threshold value, at least one overload stop command generation circuit to perform a time interval operation for a time set in accordance with the value of the overload judgment value when the value of the load current detected by the load current detector is compared with the overload judgment value set at a value smaller that the allowable threshold value and is detected to exceeds the overload judgment value and to generate a stop command to instruct the inverter to be stopped when the time interval operation is completed and inverter stop means to stop the operation of the inverter when the stop command is generated from either of the instant stop command generation circuit and the overload stop command generation circuit.